1. Field of the Invention
The present invention relates to the reduction of defects resulting from magnetron sputtering, and, more particularly to reducing and controlling the number of defects due to carbon inclusions on magnetic media.
2. Description of the Related Art
Computer disc drives commonly use components made out of thin films to store information. Typical disc drive thin film components include read-write head elements for reading and writing magnetic signals and magnetic media for writing magnetic signals onto. Conventional magnetic media is usually made by depositing a stack of thin film layers over each other as illustrated in FIG. 1A.
FIG. 1A is an illustration showing the layers of a conventional magnetic media structure including a substrate 103, a seed layer 105, a magnetic layer 107, a protective layer 109, and a lube layer 111. The first layer of the media structure is the substrate 103, which is typically made of nickel-phosphorous plated aluminum or glass that has been textured. The seed layer 105, typically made of chromium, is the first thin film deposited onto the substrate 103. The magnetic layer 107, typically made of a magnetic alloy containing cobalt (Co), platinum (Pt) and chromium (Cr), is a thin film deposited on top of the seed layer 105. The protective layer 109, typically made of carbon and hydrogen, is a thin film that is deposited on top of the magnetic layer 107. Finally the lube layer 111, typically made of a polymer containing carbon (C ) and fluorine (F) and oxygen (O), is deposited on top of the protective layer 109.
The durability and reliability of recording media is achieved primarily by the application of the protective layer 109 and the lube layer 111. The protective layer 109 is typically an amorphous film called diamond like carbon (DLC), which contains carbon and hydrogen and exhibits properties between those of graphite and diamond. Thin layers of DLC can be deposited on disks using a variety of conventional thin film deposition techniques such as ion beam deposition (IBD), plasma enhanced chemical vapor deposition (PECVD), magnetron sputtering, radio frequency sputtering or chemical vapor deposition (CVD). During the deposition process, adjusting sputtering gas mixtures of argon and hydrogen varies the concentrations of hydrogen found in the DLC. Since typical thicknesses of protective layer 109 are less than 100 Angstroms, lube layer 111 is deposited on top of the protective layer 109 for added protection, lubrication and enhanced disk drive reliability. Lube layer 111 further reduces wear of the disc due to contact with the magnetic head assembly.
Although there are several techniques available for depositing DLC films as a protective layer 109 for magnetic recording media, as previously discussed, planar magnetron is the preferred method because of its wide spread use and good resultant film properties. However, there are problems associated with using planar magnetron sputtering including low yields resulting of the high number of defects found on the disk.
FIG. 1B is an illustration showing a cross sectional side view of a conventional magnetron sputtering system including a target 110, a target erosion zone 115, a redeposition area 120, a backing plate 125, a coolant 130, magnets 135, a shunt 140, a cathode 145 and a plasma 150. Target 110 is a conventional sputtering target that is mounted to the backing plate 125 with indium. Magnets 135 are typically permanent magnets, which are used to confine plasma 150 near the surface of the target. Coolant 130 is typically water which is circulated behind backing plate 125 to cool the target while it is being sputtered. Shunt 140 diverts the magnetic field to the exterior of the target 110 causing electrons to be trapped and consequently causing sputtering of the target 110.
The sputtering process removes target material from the target erosion zone 115 and deposits that material throughout the chamber including the substrate, chamber walls and target 110. If reactive gases such as ethylene or methane are used then additional material other than the sputtered material is deposited throughout the chamber and substrate. The area on the target 110 where sputtered material gets redeposited and any film grows as a result of using reactive gasses is called the redeposition area 120. This redeposited material, located in the redeposition area 120, is sometimes ejected from the target 110 surface and bombards the substrate creating a defect, as explained in more detail below.
FIG. 1C is a block diagram showing a front view of typical planar sputtering cathode including a target 110, a target erosion zone 115 and a redeposition area 120. The target erosion zone 115, resembling a racetrack, is the area of the target 110 where material is sputtered off. The redeposition area 120 is the area on the target where carbon is redeposited during the sputtering process. Redeposition area 120 includes the rectangular area in the center of the target erosion zone 115 as well as the outer part of the target 110 between the target erosion zone 115 and the edge of the target 110.
FIG. 1D is an illustration showing a top view of a conventional magnetron sputtering system including a first chamber wall 155, a second chamber wall 160, a top view of eight planar cathode mounted sputtering targets with redeposition areas 120, a top view of eight plasma patterns 165 and a top view of a transport mechanism 170. First chamber wall and second chamber wall are both conventional walls of a vacuum chamber typically constructed out of stainless steel. The eight sputtering patterns represent the material sputtered from the erosion pattern 115 along with ionized sputtering gas atoms (argon). Transport mechanism 170 is a transportation device that moves disks or pallets full of disks in front of plasma 150 as further described with reference to FIG. 1E below.
FIG. 1E is an illustration showing a front view of one side of a conventional magnetron sputtering system including four targets 110 with erosion zones and redeposition areas and a transport 170 located within a vacuum chamber 180 as well as disks 185, a pallet 187 and a beam 191. Vacuum chamber 180 is a conventional chamber, typically made of stainless steel, that houses targets 110 and transport 170. Disks 185 are substrates 103 with seed layer 105 and magnetic layer 107 already on them and ready for depositing protective layer 109 to be deposited. Pallet 187 is typically made of aluminum and is machined to hold disks 185 in an upward position. Beam 191 is typically a stainless steel beam from which pallet 187 hangs and is transported in vacuum chamber 180.
A significant disadvantage with conventional planar magnetron sputtering techniques, such as the one described with reference to FIGS. 1A-1E, is the high number of particulates that are produced on the substrate. If too many particulates are deposited on a substrate then the substrate is defective and cannot be used. Although defects resulting from excessive particulates on a substrate can occur when sputtering any material, the problem is enhanced when sputtering carbon.
Typical carbon defects include particulates containing carbon and traces of the sputtering gases used (typically argon) that range in size from sub micron to micron in diameter. These defects, which have a high content of SP2/SP3 hybridization, are often found embedded deeply into the NiP coated aluminum substrate manifesting themselves as glide height asperities and/or thermal asperities when the magneto-resistive recording head glides over them. The rate at which these defects are generated is time dependent. New or recently resurfaced targets have a low emission rate for these defects. As the targets are sputtered, the rate increases to a maximum, and then decreases over time to a stable level. For this example of planar magnetrons, the maximum defect rate takes approximately 60 hours to be reached and then decreases over the next 120 hours of operation. The final defect rate maintains at 2-3% product loss until the targets are replaced or resurfaced.
In one model explaining the formation of particulates on a carbon surface, particulates are ejected from the redeposition area of a sputtered target and are deposited on the substrate. In this model, the defects arising out of carbon particulates increase as the redeposited material on the target increases. During the sputtering process, some of the sputtered material is redeposited back on the areas of the target material. Redeposited material is defined as the material that is sputtered off of a target and lands back on the target. This can include the target material plus other materials such as argon, hydrogen or other impurities that get commingled with the target material during the sputtering process. As the redeposited material builds up over time, stress fracturing occurs in the redeposition area 120 resulting in ejection of particulate material and a roughening of the redeposition area. Since the trajectory of these high velocity particles is random, statistically some of the particles collide with the surface of the substrate being coated. During this collision, the high velocity particles impart to the substrate sufficient energy to melt the Nickel phosphate (NiP) coating on the substrate at the contact site and to deform the surface of the substrate sufficiently to embed the particle or a proportion of the particle deeply into the NiP material. Finally, these defects manifest themselves as glide height asperities and/or thermal asperities when a magneto-resistive recording head glides over the defect, which can result in unacceptable recording media. If enough defects are found on a recording disk then the disk is rejected resulting in lower yields and higher cost.
Therefore what is needed is a system and method that reduces the amount of redeposited material on the target, consequently reducing the number of particulates ejected from the surface of the target and creating defects on the substrate. Although such a system and method for reducing substrate defects is needed in all areas of thin film growth the need is especially high in the area of recording media manufacture. Defects produced on magnetic media during the thin film deposition process are usually carried through to the finished product because subsequent processes, such as lubrication, coat and conform to the defect geometry. Defects on magnetic media often cause thermal asperities and head crashes resulting in unusable magnetic media and consequently low yields and higher cost in manufacturing magnetic media.
In order to reduce the number of defects per disk arising from particulates produced in the magnetron sputtering processes, a rotary magnetron sputtering system and method is used for depositing thin films. The rotary magnetron cathode target assembly consists of a magnet, a cylindrical cathode, a cylindrical target, a shaft for connecting to a rotary drive mechanism for rotating the assembly and a coolant. The magnet is located inside the cylindrical cathode and remains stationary as the cathode and target rotate around it. The cathode and target are coupled to the shaft which is attached to a rotary drive mechanism that rotates the shaft and coupled cathode and target.
The method of using the rotary magnetron cathode to reduce the number of defects per disk includes igniting a plasma at the surface of the target causing the target surface closest to the magnet and exposed to the plasma to be sputtered off. Next, the target and cathode are rotated around the shaft, the magnet remains stationary. The stationary magnet forces the plasma to remain stationary as the target moves around. Therefore, rotating the cathode and target about the shaft produces the effect of sweeping the target surface in front of the plasma so that only the portion of the target surface that is exposed to the plasma is sputtered off. This prevents build up of redeposited material because the entire surface gets sputtered off. As the target surface rotates, the material that is redeposited onto the surface is again sputtered off as that portion of the surface with redeposited material enters the plasma. The effect of this rotary cathode target assembly is that the entire surface is repeatedly being sputtered off so that redeposited material is not allowed to get so thick that it eventually dislodges from the surface. This dislodged material then enters the plasma where it is superheated and explodes into smaller high-energy particles that collide and embed into the disk causing a defect on the disk. This method of depositing carbon onto disks prevents the redeposited material from dislodging from the surface and entering the plasma.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.